The present invention relates to a ceramic heater apparatus having metal electrodes and, more particularly, to a plate-like ceramic heater which is used as a regenerative heater for a diesel particulate filter and improved that metal electrode attachments are so set as to feed sufficient current capacity without remaining stress.
Ceramic heaters possess excellent heat resistance and corrosion resistance, in comparison with a metallic heater, and can be used in the type of severe conditions which a metallic heater could not withstand, and various utilities have been developed.
When a ceramic heater is intended to be used, for example, as a regenerative heater of a particulate collecting filter in a diesel engine, such a heater must be able to uniformly heat the relatively large surface area of the filter, and is therefore formed in a plate shape which corresponds to the shape of the filter surface.
The plate-like ceramic heater has drawbacks in use in the construction of a bonding portion of a ceramic heater body to metallic portions of electrodes for supplying electric power for heating. To this end, the bonding portion is metallized to attain a contact surface which is able to supply sufficient power in the bonding of the electrodes with a metal material.
FIGS. 12A-12C show the electrode leading portions of a regenerative ceramic heater of a conventional diesel particulate collecting filter. Ceramic heater body 70 is formed in a plate shape, and an electrode 71, made of a metal material, is bonded to heater body 70. Metallized layer 72, formed on the surface of body 70, corresponds to an electrode bonding surface portion to which electrode 71 is bonded by means of a brazing material 73.
The surface portion of electrode 71 is formed in a plate shape, so as to sufficiently provide the bonding area of electrode 71 to body 70, thereby setting a large current capacity. A lead 74 is connected to electrode 71, for supplying electric power thereto.
Ceramic heater body 70 and metal electrode 71 have, however, large thermal expansion coefficient difference. Large bonding area of electrode 71 to heater body 70 causes the residual tensile stress to increase after brazing. Repetitive thermal stresses immediately after brazing or due to repetition of power supply introduces a crack 75 as shown in FIG. 12C in heater body 70.